Hello,
You should run at least 30% coolant OR a corrosion inhibitor, check with the local Land Rover agent for the stuff they use in the all alloy V-8 cooling system. The slow leak is probably a failing radiator, a hose would have blown by now in the tropics. Do not discount the heater matrix, if still plumbed into the system. You do not specify chassis type(W123 or 124?)
Hope that helps.

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Nachi11744